The pneumoviruses include many important human and animal pathogens, including human respiratory syncytial virus (hRSV), bovine RSV, human metapneumovirus (hMPV), avian MPV, and pneumovirus of mice. Among these viruses, hRSV and hMPV are the leading causes of acute respiratory tract infection in infants and children. Despite major efforts, there is no antiviral or vaccine to help combat these diseases.
First discovered in 1956 as a lower respiratory tract pathogen of children in their first year of life, human respiratory syncytial virus is an enveloped, negative-sense single-stranded RNA virus belonging to the Pneumovirus genus within the Pneumovirinae subfamily of the family Paramyxoviridae. hRSV is a major cause of lower respiratory tract infections, most commonly resulting in mild respiratory tract disease. However, infection with hRSV may result in severe bronchiolitis and pneumonia. In industrialized countries, hRSV accounts for up to 70% of hospitalized bronchiolitis cases. Worldwide, hRSV is estimated to cause 199,000 deaths annually in children less than 5 years.
Bovine respiratory syncytial virus (bRSV) is closely related to hRSV, although there is no evidence that cross-species transmission between cattle and humans occurs. bRSV is considered a component of the bovine respiratory disease complex, and infects cattle of all ages, although suckling calves often experience the most severe disease. Serologic surveys indicate that bRSV is a very common virus in cattle populations worldwide.
Human metapneumovirus, first characterized in 2001 in the Netherlands, belongs to the Metapneumovirus genus within the Pneumovirinae subfamily of the family Paramyxoviridae. Soon after its discovery, hMPV was recognized as a globally prevalent pathogen, likely having been causing respiratory illnesses for at least 50 years worldwide. A negative-sense single-stranded RNA virus, hMPV has been isolated from individuals of all ages with acute respiratory tract infection, especially in infants, children, the elderly, and immunocompromised individuals. hMPV infection is recognized as a leading cause of respiratory tract infection in the first years of life with symptoms similar to that of hRSV infection, including mild respiratory problems to severe coughs, bronchiolitis, and pneumonia. Transmission likely occurs through contact with contaminated secretions, and via droplet, aerosol, or fomite vectors. Epidemiological studies suggest that 5 to 15% of all respiratory tract infection in infants and young children are caused by hMPV, a proportion second only to that of hRSV.
The only other member in the genus Metapneumovirus is avian metapneumovirus (aMPV), also known as avian pneumovirus or Turkey Rhinotracheitis, is an economically important pathogen that causes acute respiratory disease in turkeys, and has been associated with swollen head syndrome in broiler chicken breeds as well as egg production losses in laying chicken breeds. First detected in turkeys in South Africa in the late 1970s, aMPV has since spread to all major poultry-producing areas in the world, except for Australia. In addition to affecting turkeys and chickens, aMPV has been detected in pheasants, Muscovy ducks, and guinea fowl. Epidemiologic studies provide evidence for the circulation of aMPV in wild birds, particularly water-associated species. Experimental studies have shown that turkeys may also be susceptible to hMPV. Infection with aMPV is often complicated by secondary bacterial infections, leading to high economic losses.
The genome of hMPV is a non-segmented negative-sense (NNS) RNA, with size ranging from 13,280 to 13,378 nucleotides, and contains 8 genes which encode for 9 proteins in the order of 3′-N-P-M-F-M2-SH-G-L-5′. Like all NNS RNA viruses, the genomic RNA is completely encapsulated by nucleoprotein (N), forming the N-RNA complex that serves as a template for genome replication and mRNA transcription. During replication, the RNA dependent RNA polymerase (RdRp) enters at the extreme 3′ end of the genome and synthesizes full-length complementary antigenome, which in turn serves as template for synthesis of full-length progeny genome. During transcription, RdRp copies the genomic RNA template to synthesize a short uncapped leader RNA, and capped, methylated, and polyadenylated mRNAs that encode all viral proteins. The components of RdRp of Paramyxovirinae subfamily (family Paramyxoviridae), Rhabdoviridae, and Bornaviridae include the large (L) protein catalytic subunit and phosphoprotein (P) cofactor. The RdRp of the Pneumovirinae subfamily of the family Paramyxoviridae requires the M2-1 protein as an additional cofactor, whereas the Filoviridae (such as Ebola virus and Marburg virus) polymerase requires VP30 as an additional cofactor. Both the M2-1 protein of pneumoviruses and VP30 protein of filoviruses are typical zinc binding proteins thought to play many critical regulatory roles in RNA synthesis and processing via poorly understood mechanisms.
Metal ions were shown to be integrated in several gene regulatory proteins as early as the 1970s. Among them, zinc is an important structural component of proteins involved in nucleic acid binding and gene regulation. Zinc binding motifs, including CCHH, CCHC and CCCH, are often involved in transcriptional and translational processes. Many viruses encode their own zinc binding proteins that regulate viral replication and/or pathogenesis, such as NS5A protein of hepatitis C, almost all retroviral nucleocapsid proteins, which comprise one or two copies of a zinc finger motif essential for viral replication, the V proteins of many paramyxoviruses, and the VP30 protein of Ebola.
The M2-1 protein is unique to all known pneumoviruses. The current understanding of the functions of M2-1 proteins comes predominantly from studies of the hRSV M2-1 protein. The hRSV M2-1 functions as a transcriptional elongation factor and anti-terminator that enhances read-through of intergenic junctions. Thus, M2-1 is essential for synthesis of full-length mRNAs and polycistronic mRNAs. The hRSV M2-1 was found to be an RNA binding protein, although its RNA binding specificity is controversial. Recent NMR studies showed that the RSV M2-1 core domain preferentially recognizes poly-A tails of viral mRNAs. M2-1 likely binds nascent mRNA transcripts, preventing premature termination through stabilization of the transcription complex and inhibition of RNA secondary structure formation. Additionally, hRSV M2-1 was shown to interact with the N, P, and L proteins.